System and method of identifying path of residual current flow through an intelligent power strip

ABSTRACT

A system and method is disclosed for detecting a specific voltage phase, from a multiphase voltage source, and a specific outlet of an intelligent power strip, that is associated with a residual current flow. The method accomplishes this by using a system that employs a statistical time series analysis using a Pearson&#39;s correlation coefficient calculation to measure the linear dependence between the discretely sampled residual current waveform and each phase and outlet&#39;s discretely sampled current waveforms, in turn. A residual current as low as 1 mA can be accurately measured and its associated voltage phase source, as well as which outlet of an intelligent power strip it flows out of, can be reliably determined.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.62/443,308, filed on Jan. 6, 2017. The entire disclosure of the aboveapplication is incorporated herein by reference.

FIELD

The present disclosure relates to a system and method of identifying apath of residual current through an Intelligent Power Strip (IPS).

BACKGROUND

The statements in this section merely provide background informationrelated to the present disclosure and may not constitute prior art.

Residual currents caused by the failure of insulation can constitute asignificant risk to safety in electrical systems. Using an appropriateprotective concept it is possible to detect residual currents,discover/eliminate insulation faults quickly, and therefore ensure theavailability of the system.

The acronym “RCM” stands for “Residual Current Monitoring” and means themonitoring of residual currents in electrical systems. While the currentsensors in the electrical systems referred to herein are AC currentsensors, if different current sensor types or configurations are used,DC components could also be accurately measured using the samemonitoring method. The residual current is calculated as the non-zerosum of the currents flowing through all current-carrying conductors,apart from the protective earth (PE), which feed into the electricalsystem. Residual currents are typically the result of insulation faults,residual currents or electromagnetic compatibility (EMC) filter residualcurrents, for example. While RCD devices (residual current circuitbreakers) switch off the power supply in the event of a certain residualcurrent being exceeded, RCM measuring devices indicate the actual value,record the long-term development and report when the measured valueexceeds a critical value. This information can also be used in order toswitch off the power supply via external switching devices (contactors,relays). Through the use of residual current measuring (RCM) devices, itis possible to detect and report residual currents in a timely manner.This makes it possible to initiate counter measures within asufficiently short time so that it is not necessary to switch the systemoff. This facilitates the implementation of measures in the event ofslowly deteriorating insulation values or steadily rising residualcurrents—caused for example by aging insulation—before the system isswitched off.

A power strip, often referred to as a “power distribution unit” (“PDU”),is typically used in a data center environment, with one or more unitsinstalled in racks arranged in rows, to power Internet Technologyequipment (ITE). A single PDU may provide power to dozens of devices perrack via outlet sockets, and an “intelligent” rack PDU employed in adata center can measure and control the loads. A powered device, e.g.,server or network switch, has one or more internal switched mode powersupplies that on occasion may fail prematurely for various reasons,e.g., exposure to excessively high-temperature which can degradeelectronic component lifetime. Also, the integrity of the TNS systems(Terra Neutral Separate) may become inadvertently disconnected or fail.These failures may develop suddenly or gradually over time. The failuremode may result from compromised or complete breakdown of componentinsulation spacings, resulting in a lower impedance conduction pathbetween line voltages of the device's power supply to protective earthground. While the residual current that flows through the protectiveearth ground is not of sufficient magnitude to trip the unit's branchovercurrent protection device, only 30 mA can become a safety hazard toanyone touching the chassis. While it is critical to identify andprovide an alert when a condition of excessive ground residual currentdevelops, it is also important that the offending device can be quicklyisolated and removed from the power distribution to maintainhigh-availability of other systems components.

One particular known technique for measuring residual current involvesusing a sensitive current transformer to detect and/or measure theresidual current or current that does not flow back on the return paththrough an intelligent power strip. Although this method is able todetect that a residual current condition exists, it is unable toidentify which one of a plurality of voltages of a multiphase voltagesupply is sourcing the residual current condition, as well as whichspecific AC outlet of the PDU is associated with the residual currentcondition.

SUMMARY

This section provides a general summary of the disclosure, and is not acomprehensive disclosure of its full scope or all of its features.

In one aspect the present disclosure relates to a method for detecting aresidual current flowing through an intelligent power strip having atleast one alternating current (AC) power outlet. The method may compriseobtaining current sensing information and performing ananalog-to-digital (ADC) conversion of the current sensing information toobtain ADC samples. The method may further include managing a timeseries collection of the ADC samples as residual and outlet currentwaveform samples, and then calculating residual current (RC) RMS valuesfor the obtained residual current samples. The method may furtherinclude calculating phase current from the aggregation of the outletcurrent waveform samples having the same phase, and then calculatingPearson's correlation coefficients for variables relating to residualcurrent and the phase outlet current waveforms. The method may furtherinclude calculating Pearson's correlation coefficients for the variablesrelating to residual current and individual outlet currents waveformsfrom each AC power outlet, and calculating the phase having a maximumpositive Pearson's correlation when the residual current RMS is greaterthan a predetermined residual current RMS threshold. Finally the methodmay include determining an individual one of the AC power outlets havingthe maximum Pearson's correlation when the residual is greater than thepredetermined RC threshold.

Further areas of applicability will become apparent from the descriptionprovided herein. The description and specific examples in this summaryare intended for purposes of illustration only and are not intended tolimit the scope of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawing described herein is for illustration purposes only and isnot intended to limit the scope of the present disclosure in any way. Inthe drawing figures:

FIG. 1 is a high level block diagram of one embodiment of an apparatusin accordance with one embodiment of the present disclosure;

FIG. 2 is a high level block diagram showing in greater detail variouscomponents that may be incorporated into the microcontroller shown inFIG. 1;

FIG. 3 is a high-level flowchart summarizing operations performed by theapparatus of FIG. 1 in detecting a residual current condition, as wellas a specific voltage phase associated with the residual currentcondition. If the apparatus of FIG. 1 includes dedicated currentmetering circuitry to support measurement of current at individual ACoutlets, a residual current condition at one or more AC outlets can alsobe detected;

FIG. 4A is a detailed flowchart illustrating operations performed by theapparatus of FIG. 1 in detecting a residual current condition, as wellas a specific voltage phase; and

FIG. 4B shows additional operations that may be performed by theapparatus of FIG. 1 if the apparatus includes dedicated current meteringcircuitry to support measurement of current at individual AC outlets.

DETAILED DESCRIPTION

The following description is merely exemplary in nature and is notintended to limit the present disclosure, application, or uses. Itshould be understood that throughout the drawings, correspondingreference numerals indicate like or corresponding parts and features.

Referring to FIG. 1, one embodiment of an apparatus 10 is shown whichforms an intelligent equipment rack power distribution unit able tomonitor for residual current associated with any one or more of aplurality of rack mounted assets being powered by the apparatus 10. Forconvenience, the apparatus 10 will be referred to throughout simply as“PDU 10”.

The PDU 10 may incorporate a residual current monitor currenttransformer 11, hereinafter simply “RCMCT 11”, which is operativelycoupled to three-phase AC power from some upstream device (e.g., ACmains source, UPS, etc.) and which senses a residual current conditionaffecting any one or more of a plurality of AC outlets 10 a of the PDU.The operation of the RCMCT 11 and its connections to other components ofthe PDU 10 will be described in greater detail in the followingparagraphs.

The PDU 10 in this example may also incorporate a communications moduleRPC2 12 (hereinafter simply “RPC 12”), which may be a hot-swappable webcard which is installed in the PDU 10, and in this example may be theRPC2™ Network Interface Card available from Vertiv Co., assignee of thepresent disclosure. The RPC 12 may include a microcontroller 14 andpreferably also a non-volatile (NV) memory 16. The RPC 12 may alsoinclude a plurality of ports including, but not limited to, a LANEthernet port 18, an Expansion/Management port 20, a port 22 forcoupling to a display module (e.g., “BDM” or “Basic Display Module”available from the assignee of the present disclosure), one or more1-wire sensor ports 24, an RS-232 port 26 and a USB port 28.

The PDU 10 further may include a Rack PDU Controller (RPDUC) 30 having amicrocontroller 32 and a non-volatile memory 34, and one or more branchreceptacle controllers (BRC) 36. The RPDUC 30 receives currentinformation from the RCMCT 11. Each BRC 36 may have a complexprogrammable logic device (CPLD) 38 having a voltage and current sensingsubsystem 38 a which senses of a loss of AC input power, a plurality ofbistable relays 40, and an open circuit breaker (OCB) detectionsubsystem 42 which senses for an open circuit breaker condition. TheRPDUC 30 is in bidirectional communication with each of the BRCs 36 viaa bus 44. The RPC 12 is in bidirectional communication with the RPDUC 30via a bus 46. A reset switch 48, which is easily accessible by a uservia a faceplate of the PDU 10, is provided for enabling the user toinitiate a hard reset of the PDU.

FIG. 1 also shows a plurality of branch circuit breakers (CB) 50 thateach receives AC input power directly from the current-carryingconductors passing through the RCMCT 11. By “branch” circuit breaker itis meant that each one of the CBs 50 is typically associated with onespecific BRC 36. The OCB detection subsystem 42 monitors the CBs 50 todetect when any one or more have been tripped to an open condition. Andas explained above, each BRC 36 includes a plurality of bistable relays40, which in one specific embodiment comprise eight (8) bistable relays.However, it will be appreciated that a greater or lesser number ofbistable relays 40 could be provided per branch. Mechanical bistablerelays have coils and mechanical contacts. They can be single coil ordual coil relays. Also, more than one CB 50 may exist for each BRC 36.For example, each BRC 36 can have its bistable relays arranged in twosub banks, with a separate CB 50 associated with each sub-bank. As usedherein, each sub-bank of a BRC 36 is a branch of the BRC.

FIG. 1 also shows the plurality of AC power outlets 10 a. In thisexample, each outlet is single-phase in line-neutral or line-lineconfiguration, with its associated phase established at manufacturingtime and saved in NV memory 16. Each outlet has a first associatedoptical element 10 a 1 and a second optical element 10 a 2. Opticalelements 10 a 1 may each be an LED having a first color, for examplegreen, that indicates a status of the specific bistable relay 40associated with its specific AC outlet 10 a. The second group of opticalelements 10 a 2 may also be, for example, LEDs having a different color,for example red, for providing additional information to the user. Eachone of the green LEDs 10 a 1 may indicate, for example, that thebistable relay 40 associated with that specific AC outlet 10 a isclosed, and an extinguished green LED 10 a 1 would therefore indicatethat the associated bistable relay is open. Each AC power outlet 10 amay be used to power an associated rack mounted asset (e.g., server,network switch, etc.)

The RPDUC 30 is shown in greater detail in FIG. 2. The microcontroller32 of the RPDUC 30 may include a static random access memory 32 a(hereinafter simply “SRAM 32 a”) and a non-volatile read only memory ornon-volatile random access memory 32 b (hereinafter simply “NV memory 32b”). The SRAM 32 a may contain a plurality of circular buffers 32 a 1 tohold current waveform data samples, while the NV memory 32 b may be usedto store gain coefficients 32 b 1 and ADC skew coefficients 32 b 2. Theuse of the circular buffers 32 a 1, the gain coefficients 32 b 1 and theADC coefficients 32 b 2 will be discussed further in the followingparagraphs. And while the SRAM 32 a and NV memory 32 b in this exampleare shown as part of the microcontroller 32, it will be appreciated thatthey could be implemented as independent memory devices just as well.

The RPDUC 30 includes a voltage sensing subsystem 52 and a currentsensing subsystem 54. The subsystems 52 and 54 perform real time RMSvoltage measurements and RMS current measurements, respectively, andthus monitor the power input from the AC power source. The RCMCT 11 isalso in communication with the current sensing subsystem 54. Three phaseLEDs 55 a-55 c may be incorporated and/or operatively coupled to theRPDUC 30 that indicate the voltage and current conditions of each phaseof the AC input, including the residual current condition. The monitoredpower information may be shared with the RPC 12 via bus 44. As notedabove, the voltage and current sensing subsystem 38 a of each BRC 36also monitors for a loss of AC input power, so in this regard there isredundancy of this feature in the RPDUC 30 and the BRCs 36. The currentsensing subsystem 54 of the RPDUC 30 receives an input current signalfrom each of the branch BRCs (collectively labeled for simplicity inFIG. 2 with number 36) which it uses to perform its current sensingfunction. Each branch BRC 36 also includes a plurality of currenttransformers (CTs) 56 for independently measuring a current being drawnby the AC outlets 10 a associated with each branch of bistable relays40. The signals from each branch CT 56 are input to the current sensingsubsystem 54 for analysis.

The RPC 12 shown in FIG. 1 manages, monitors and reports informationabout PDU 10 energy metering and power distribution status obtained fromthe RPDUC 30 to networked software clients. The RPDUC 30 providessupport for the energy metering measurements and calculations, controlmanagement, and communications interfaces to the RPC 12, as describedabove. The RPDUC 30 communicates with each BRC 36 and, except upon powerloss, controls the bistable relays 40 of each BRC by sending commandmessages to each BRC to independently control each one of its associatedbistable relays 40.

The BRC 36, and more particularly its CPLD 38, directly controls itsbistable relays 40. The BRC 36 also senses individual LED outletoperational status, and loss of an AC input power signal via linefrequency monitoring performed by the voltage and current detectionsubsystem 38 a, as well as using the OCB subsystem 42 to detect for anopen circuit breaker condition. The bistable relays 40 of each BRC 36 inthis example require a nominal 16 msec pulse to their coils to changestates, that is, to open or close their contacts. A reference herein toa bistable relay being “open” means that its contacts are open and poweris off or interrupted at the outlet 10 a to which the bistable relayswitches power. As used herein, “power up”, “power down”, “powerfailure”, and “power cycle” refer to specific conditions of input ACline voltage, which is the AC power provided to the outlets 10 a throughthe bistable relays 40 of each BRC 36. The term “Configured state”, whenused in connection with the bistable relays 40, means the state that agiven bistable relay is configured to be in (i.e., open or closed) whenpower is on. For the purpose of the present disclosure, it may beunderstood that the term “configured state” means that the bistablerelays 40 will, after the PDU 10 is powered up, have closed contacts inorder to switch power on at the AC outlets 10 a.”

The RPC 12 commands the RPDUC 30 via a SMBus (I2C) communication bus,bus 46 in FIG. 1 in this example, which in turn, commands the BRC 36 viaa SPI communication bus, which is bus 44 in this example, to configurethe relay state of each bistable relay 40. The RPDUC 30 is capable ofautonomous behavior without RPC 12 commands. The one or more BRCs 36 areeach capable of autonomous behavior without RPDUC 30 commands.

The PDU 10 and its method of operation significantly extend the abilityof traditional systems and methods for measuring residual current byincorporating statistical analysis. The statistical analysis is used toinfer which voltage phase of the AC input voltage, and which one of theAC outlets 10 a (FIG. 1) of the PDU 10, is affected by the path of aresidual current. In this manner it is possible to quickly detect whichone of a plurality of rack assets connected to the AC outlets 10 a ofthe PDU 10 is the faulty device giving rise to the residual current flowso that the faulty device can be quickly disconnected from the PDU. Thisenables the remaining, properly functioning, devices operating on thePDU 10 to continue in their operation without the need to independentlypower down each of the properly functioning devices coupled to the PDUin order to identify the offending device.

Referring to FIG. 3, a flowchart 300 is shown providing a high-levelsummary of the operations that may be performed by the PDU 10 incarrying out the detection of a residual current associated with any ACoutlet 10 a of the PDU 10. FIG. 3 also provides a list of the discreteoperations that may be performed to carry out each of the high-leveloperations, to be discussed below with reference to FIG. 4. It should benoted that operations illustrated in the flowchart 300 may be performedby the microcontroller 32 of the RPDUC 30 of the PDU 10, but forconvenience it will be stated hereinafter that an operation may beperformed by the PDU 10 or by the microcontroller 32.

As an overview of the various operations shown in FIG. 3, it will beunderstood that the PDU 10 detects the voltage phase in which theresidual current path is flowing. If the PDU 10 supports individualcurrent measurements at the outlets, then the PDU 10 also detects the ACoutlet 10 a through which the residual current path is flowing. The PDU10 accomplishes this by using statistical time series analysis alongwith a Pearson's correlation coefficient calculation to measure thelinear dependence between the discretely sampled residual currentwaveform for each phase and the aggregation of each AC outlet's 10 adiscretely sampled current waveforms calculated per phase, in turn. Theper-phase aggregation may result from current measurements by a singlesensor dedicated to a set of same-phased outlets or by a plurality ofsensors per each outlet. The Pearson's correlation coefficient is thecovariance of the compared values divided by the product of theirstandard deviations. A residual current as low as 1 mA can be accuratelymeasured/detected along with its associated voltage phase source. Fromthis information it can be reliably determined which AC outlet 10 a theresidual current is flowing out of. The presence of a residual currentis constantly measured and monitored for in real-time. When a residualcurrent threshold level is exceeded, an audible alarm may be sounded, anevent notification may be delivered, and a visual indicator of theassociated phase may be flashed on the phase LED 55 a-55 c and/or on theLED corresponding to an outlet of the intelligent power strip, if thePDU 10 is equipped with dedicated current metering circuitry per outlet,until the residual current path is eliminated or the alarm threshold isincreased.

Referring further to FIG. 3, the methodology employed by the PDU 10 maybegin at operation 302, with analog-to-digital conversion (ADC) and timeseries collection of residual and outlet current waveform samples. Thisis accomplished by means of discrete operation 106, to be discussedbelow. At operation 304, the root mean square (RMS) value of theresidual current may be calculated, by means of discrete operations 108,110, 112, 114, 116, 118, 120, 140, 142, 144, 146, 148, 150, and 184. Atoperation 306, the currents of individual outlets and aggregated outlet(phase) currents may be calculated. This may be accomplished by means ofdiscrete operations 122, 124, 126, 128, 130, 132, 134, 136, 140, and184. At operation 308, discrete operations 180, 182, and 152 may beperformed in order to calculate Pearson's correlation coefficients forresidual and phase currents. At operation 310, Pearson's correlationcoefficient may be calculated for RC and individual outlet currents bymeans of discrete operations 154, 182, and 156. It should be noted thatoperation 310 supports both in-phase and quadrature evaluation andincremental phase shift up to 15 degrees. At operation 312, the phasehaving the maximum Pearson's correlation is calculated when the residualcurrent exceeds the threshold; this is accomplished by discreteoperation 158. If the PDU 10 is equipped with dedicated current meteringcircuitry for each outlet 10 a, operation 314 may be performed todetermine the outlet having the maximum Pearson's correlation when theresidual current exceeds the threshold. This is accomplished by thesequence of discrete operations 160, 162, 163, 164, 166, 168, 170, 172,174, 176, and 178.

Referring to FIG. 4A, a flowchart 100 is shown illustrating variousoperations that may be performed by the PDU 10 in carrying out thedetection of a residual current associated with any AC outlet 10 a ofthe PDU 10. It should be noted that operations illustrated in theflowchart 100 may be performed by the microcontroller 32 of the RPDUC 30of the PDU 10, but for convenience it will be stated hereinafter that anoperation may be performed by the PDU 10 or by the microcontroller 32.

Referring further to FIG. 4A, the methodology employed by the PDU 10 maybegin at operation 102 where phase analysis variables α and φ areinitialized, and at operation 104 the ADC skews K_(r) and K_(o) may bepre-calculated. The ADC skews K_(r) (for each residual current ADCchannel) and K_(o) (for each outlet current channel) are pre-calculatedtime delays, representing the delta time between successive samplescaptured one after another; if each sample duration is approximately nmicroseconds, with m outlet currents the last sample is captured after(m−1)*n μsec. The skew adjustment uses a linear extrapolation toestimate an extrapolated normalized current waveform sample, as if all moutlet currents were sampled simultaneously by microcontroller 32. Atoperation 106 a time series of sampled current waveforms received overan AC power line cycle may be collected.

At operation 108 the starting index n of the s_(r) circular buffer 32 a1 for RCM current waveform samples is initialized in accordance withEquation 1 below.

n←ϕ _(α)=α*90+φ

N←n+s_(rsize)  Equation 1

At operation 110 a test may then be conducted on the s_(r) buffer index(n<N) to determine when the time series has been completely processed.If the test produces a “Yes” answer, then at operation 112 the residualcurrent sample is filtered by the exponentially weighted moving averageand copied into buffer i_(r) in accordance with Equation 2 below. Thevalue of β₁ (in this example, 0.05) is dependent upon sample rate (inthis example, the controller samples 64 times per line cycle) andprovides acceptable smoothing characteristics for 1 ma precision whileremaining responsive to changing conditions.

s_(r)[n]←s_(r)[n]+β_(1*)(s_(r)[n]−s_(r)[n−1])

i _(r′) =s _(r) [n], i _(r″) =s _(r) [n−1]  Equation 2

At operation 114, corrections to remove electronic and signal offsetsand apply calibrated scalars determined at manufacturing time are thencalculated for I_(r), ADC skew, mean and gain, as indicated by Equation3 below.

i _(r) =G _(r*)(i _(r′) −K _(r)/(i _(r′) −i _(r″))−i _(rμ))  Equation 3

At operation 116, the residual current sample minimum/maximum peaks maybe determined, as indicated by Equation 4 below. These may be saved inorder to later determine the half-wave symmetry of the current waveform(i.e., full-wave or half-wave characteristic).

i_(r∧)←i_(r∧)>i_(r′∧)

i_(r∨)←i_(r∨)<i_(r′∨)  Equation 4

At operation 118, the microcontroller 32 may then calculate the residualcurrent sample integral in accordance with Equation 5 below. Because azero-valued integral is expected over a line cycle of a half-wavesymmetrical periodic waveform, operation 118 enables precise correctionof creeping offset errors.

i_(rΣ)=Σi_(r)  Equation 5

At operation 120 the residual current sample may be corrected with anoffset depending upon its detected full-wave or half-wavecharacteristic, in accordance with Equation 6 below. If full-wave, nogross offset adjustment occurs at operation 120. If half-wave, then agross offset adjustment is made to reposition the flat baseline of theresidual current waveform at the mathematical zero offset positon sothat a true RMS calculation can be made.

I_(r)←i_(r)+i_(rΔ)  Equation 6

At operation 122, the microcontroller 32 may initialize the startingindex (m=0) of i_(o) buffers for outlet current waveform samples, whereM=number of AC outlets (i.e., number of AC outlets 10 a in FIG. 1).

At operation 124 the microcontroller 32 may then test the i_(o) bufferindex (m<M) to check if another AC outlet current sample needs to beprocessed. If this test produces a “Yes” answer, then operation 126 isperformed, where the AC outlet current sample is filtered and copiedinto the buffer i_(o) in accordance with Equation 7 below.

s_(o)[n]←s_(o)[n]+β_(1*)(s_(o)[n]−s_(o)[n−1])

i _(o′) =s _(o) [n], i _(o″) =s _(o) [n−1]  Equation 7

At operation 128, the microcontroller 32 may then correct for i_(o)[m]ADC skew, mean and gain in accordance with Equation 8 below.

I _(o) [m]=G _(o*)(i _(o′) −K _(o) [m]/(i _(o′) −i _(o″))−i _(oμ)[m])  Equation 8

At operation 130 the microcontroller 32 may then calculate phasecurrents (I_(L1), I_(L2), and I_(L3) in this example) for each phase ofthe AC input (phases L1, L2, and L3 in this example) by aggregating eachof the M outlet current waveform samples. This operation is performed inaccordance with Equation 9 as shown below.

I_(L1)=Σi_(o)[m]∀m∈L1

I_(L2)=Σi_(o)[m]∀m∈L2

I_(L3)=Σi_(o)[m]∀m∈L3  Equation 9

At operation 132, the microcontroller 32 may then calculate the outletcurrent sample integral in accordance with Equation 10 below. Because azero-valued integral is expected over a line cycle of a half-wavesymmetrical, periodic waveform, this operation enables precisecorrection of creeping offset errors due to small measurementimprecision.

i_(oΣ)[m]=Σi_(o)[m]  Equation 10

At operation 134 the microcontroller 32 may then calculate the Pearson'sterms for the outlet current sample using the corrected residual currentvalues from operation 120. This operation is performed in accordancewith Equation 11 below:

ΣI_(r*)I_(o)[m], Σi_(o) ²[m]  Equation 11

At operation 136 the outlet index is incremented (m←m+1), and operationsfrom 124 may then be re-performed.

If the i_(o) buffer index test at operation 124 produces a “No” answer,this means that an output current sample has been collected for each oneof the AC outlets 10 a, and the microcontroller 32 may then executeoperation 180, in which the microcontroller calculates the Pearson'sterms for the phase current samples, in accordance with Equation 12below.

ΣI_(r*)I_(L1), ΣI_(L1) ²

ΣI_(r*)I_(L2,) ΣI_(L2) ²

ΣI_(r*)I_(L3,) ΣI_(L3) ²  Equation 12

At operation 182 the microcontroller 32 may then calculate the Pearson'sterm for the residual current sample (ΣI_(r) ²). The sample index(n←n+1) may then be incremented at operation 184, and operation 110 maybe repeated, and operations from 110 may then be re-performed.

If the s_(r) buffer index test at operation 110 (n<N) produces a “No”answer, then at operation 138 the time series has been completelyprocessed and the microcontroller 32 may calculate the RMS residualcurrent in accordance with Equation 13 below.

I _(rRMS)=√(ΣI _(r) ²/(N−n))  Equation 13

The microcontroller 32 may then calculate the running mean of residualand outlet current samples from weighted sample integrals calculated inoperations 118 and 132, as indicated at operation 140, in accordancewith Equation 14 below.

i _(rμ)+=β_(2*) i _(rρ)

I _(oμ) [m]+=β _(2*) i _(oΣ) [m]  Equation 14

The microcontroller 32 may then initialize the half/full wave offset(i_(rΔ)=0) as indicated at operation 142, and then perform a test of thepositive half-wave residual current at operation 144 by magnitudecomparison of half-wave symmetry, in accordance with Equation 15 below.

(|i _(r∨) −i _(rμ)|)>β_(3*)(|i _(r∧) −i _(rμ)|)  Equation 15

If the test at operation 144 produces a “Yes” answer, then at operation146 the microcontroller 32 calculates the positive offset in accordancewith Equation 16 below.

i _(rΔ)=+(|i _(r∧) −i _(rμ)|)  Equation 16

If the test at operation 144 produces a “No” answer or if operation 146has been performed, then at operation 148 the microcontroller 32 teststhe negative half-wave residual current in accordance with Equation 17.

(|i _(r∧) |−i _(rμ))>β_(3*)(|i _(r∨) −i _(rμ)|)  Equation 17

If the test at operation 148 produces a “Yes” answer, then at operation150 the microcontroller 32 calculates the negative offset in accordancewith Equation 18 below.

i _(rΔ)=−(|i _(r∨) −i _(rμ)|)  Equation 18

If either operation 148 produces a “No” answer or if operation 150 hasbeen performed, then at operation 152 the microcontroller 32 calculatesthe Pearson's correlation coefficient for the phase currents inaccordance with Equation 19 below.

r _(L1) =ΣI _(r*) I _(L1)/(ΣI _(L1) ² _(*) ΣI _(r) ²)∀ϕ₀

r _(L2) =ΣI _(r*) I _(L2)/(ΣI _(L2) ² _(*) ΣI _(r) ²)∀ϕ₀

r _(L3) =ΣI _(r*) I _(L3)/(ΣI _(L3) ² _(*) ΣI _(r) ²)∀ϕ₀  Equation 19

At operation 154 the microcontroller 32 calculates the Pearson'scorrelation coefficients for the outlet currents in accordance withEquation 20 below.

r _(oϕ0) ΣI _(r*) I _(o)/(ΣI _(o) ² _(*) ΣI _(r) ²)∀ϕ₀

r _(oϕ1) ΣI _(r*) I _(o)/(ΣI _(o) ² _(*) ΣI _(r) ²)∀ϕ₁  wquation 20

At operation 156 the microcontroller 32 calculates the RMS Pearson'scorrelation coefficient for outlet currents, in accordance with Equation21 below.

r _(oRMS)=√((r _(oϕ0))²+(r _(oϕ1))²)  Equation 21

At operation 158 the microcontroller 32 then sorts correlated phasecurrents (e.g., r_(L∨)=r_(L1)>r_(L2)>r_(L3)) in order to determine thelargest positive max correlation value (r_(Lmax)).

With reference to FIG. 4B, it will be understood that the sequence ofoperations from 160 through 178 is performed only if the PDU 10 includesdedicated current metering circuitry to support measurement of currentat individual AC outlets. If this is the case, then in FIG. 4B atoperation 160, the microcontroller 32 then sorts correlated outletcurrents (e.g., r_(o∨)=r_(oRMS(1))>r_(oRMS(2)) . . . >r_(oRMS(m))) inorder to determine the largest positive max correlation value(r_(omax)).

At operation 162 the microcontroller 32 then performs a test of theresidual current threshold (I_(rRMS)>I_(rthres)). If this test producesa “Yes” answer, indicating the residual current threshold has beenexceeded, then at operation 163 the value of the phase variable α istested (α=1) for the presence of a quarter cycle or quadrature phaseshift. If this test produces a “Yes” answer, then at then at operation164 the microcontroller 32 tests the maximum phase shift angle (φ=15°)to determine if the phase shift angle has reached 15°. This is done inorder to evaluate the max correlation over a small range, in case aresistive fault has a small parallel stray capacitance which wouldproduce a corresponding incremental phase shift of the residual currentwaveform with respect to the outlet current waveform. A small powerfactor difference between outlet loads would result in a larger, morediscriminating max correlation and therefore even more reliable outletdetection. If this test produces a “No” answer, then at operation 166the microcontroller 32 tests to determine if the maximum outletcorrelation (r_(o∨)>r_(o∨′)) has been exceeded. If the test at operation166 produces a “Yes” answer, then at operation 168 the microcontroller32 saves the new max outlet correlation and its phase shift (φ′←φ andr_(o∨′)←r_(o∨)). After operation 168, or if the test at operation 166produces a “No” answer, then at operation 170 the microcontroller 32increments the phase shift (φ←φ+1).

If the residual current threshold test at operation 162 produces a “No”answer, then at operation 172 the microcontroller 32 resets the phaseshift max correlation (φ=0, φ′=0, and r_(o∨′)=0). Once either ofoperations 170 or 172 is performed, or if the test at operation 163produces a “No” answer, then at operation 174 the microcontroller 32 mayalternate quarter-cycle or quadrature phase shifts (α←(α+1) mod 1). Theoutlet current waveforms may have non-linear sinusoidal characteristicsand/or be phase shifted for reactive loads, producing a non-unity powerfactor. Thus, considering the in-phase and quadrature calculations intesting for max correlation produces a larger discriminating maxcorrelation result. After operation 174 is performed, then operations108 and 110 may be repeated.

If the test for max phase shift angle at operation 164 produces a “Yes”answer, then at operation 176 the microcontroller 32 sets the phaseshift (φ←φ′), and then at operation 178 reports the RMS residual current(I_(rRMS)), the outlet correlation (r_(o∨)), and the max outletcorrelation (r_(o∨)). Operations 108 and 110 may then be repeated.

The PDU 10 and its method of operation as described herein do notrequire costly circuitry dedicated per each phase and/or outlet todirectly measure residual current flow. A particularly important anduseful property of the PDU 10 and its method of operation is that it isinvariant to scale or magnitude of the compared values. A set ofcorrelation values are calculated between −1 and +1, where a valueclosest to +1 means the linear dependence and phase matching is thehighest, and therefore, the most likely path of the residual currentflow. A residual current as low as 1 mA can be accurately measured andits associated voltage phase source and flow through outlet can bereliably determined.

The various embodiments of the present disclosure provide a significantadvantage over prior art methods that can merely measure for a residualcurrent condition. The Vertiv MPH2 rack PDU controller firmware, forexample, may be used to support the methodology described herein.

The various embodiments and methodology of the present disclosurepresents a lower cost solution when compared to directly measuringdifferential currents at each phase and/or outlet. The variousembodiments and methodology of the present disclosure offer highlysensitive detection of small phase differences of 0.5% power factorbetween compared current waveforms. The methodology of the presentdisclosure automatically discriminates the most likely residual currentpath external to the unit, so the powered device causing the fault canbe quickly removed to minimize down times and reduce the associatedcosts, rather than being required to manually identify the residualcurrent path by a trial and error method.

The various embodiments of the PDU 10 and its method of operationfurther facilitate a preventative maintenance program through theability to obtain additional indirect information about the health ofinsulation system(s) associated with electrical cabling being used, andthus may help prevent unanticipated downtime of all types ofelectrically powered devices. The PDU 10 and its method of operation, byits ability to quickly detect residual currents and identify specific ACoutlets that such currents are associated with, may also provide ameasure of fire protection for a facility in which the presentdisclosure is used.

While various embodiments have been described, those skilled in the artwill recognize modifications or variations which might be made withoutdeparting from the present disclosure. The examples illustrate thevarious embodiments and are not intended to limit the presentdisclosure.

Therefore, the description and claims should be interpreted liberallywith only such limitation as is necessary in view of the pertinent priorart.

What is claimed is:
 1. A method for detecting a residual current flowingthrough an intelligent power strip having at least one alternatingcurrent (AC) power outlet, the method comprising: obtaining currentsensing information; performing an analog-to-digital (ADC) conversion ofthe current sensing information to obtain ADC samples; managing a timeseries collection of the ADC samples as residual and outlet currentwaveform samples; calculating residual current (RC) RMS values for theobtained residual current samples; calculating phase current from theaggregation of the outlet current waveform samples having the samephase; calculating Pearson's correlation coefficients for variablesrelating to residual current and the phase outlet current waveforms;calculating Pearson's correlation coefficients for the variablesrelating to residual current and individual outlet currents waveformsfrom each AC power outlet; calculating the phase having a maximumpositive Pearson's correlation when the residual current RMS is greaterthan a predetermined residual current RMS threshold; and determining anindividual one of the AC power outlets having the maximum Pearson'scorrelation when the residual is greater than the predetermined RCthreshold.